A semiconductor device having a common source structure and method of manufacturing the same are provided. In one embodiment, the method includes: forming a plurality of gate lines on a semiconductor substrate, each constituted by a floating gate, a dielectric layer, and a control gate having a line form; forming a first dielectric layer on the semiconductor substrate including the gate line; forming a trench having the line form in the first dielectric layer, wherein the trench exposes the semiconductor substrate between the gate lines; and forming a common source in the trench. According to an embodiment, the common source is implemented as a poly line in the trench. Therefore, etching the substrate to provide a trench for a common source can be excluded. Accordingly, it is possible to inhibit the common source from being opened due to a remaining material in a trench, and reduce damage to the semiconductor substrate.